1. Field of the Invention
The present invention relates to a substrate attaching device and a substrate attaching method, and especially to a device typically used for combining two substrates such as those that form a framework of a liquid crystal display (LCD) cell.
2. General Background
An LCD cell generally comprises two glass substrates, a peripheral sealant, and a plurality of liquid crystal molecules retained in a space defined between the substrates and the sealant. The sealant is first printed on one of the glass substrates, and is then adhered to the other glass substrate. The substrates and the sealant cooperatively form the space therebetween, and then the liquid crystal molecules are filled into the space.
There are generally two methods used for filling the liquid crystal molecules into the space. The first method is to fill the liquid crystal molecules through filling ports. This method comprises the following steps: firstly, printing a sealant on a first glass substrate, wherein the sealant is rectangular and has one or more gaps that function as filling ports; secondly, combining a second glass substrate with the first glass substrate and curing the sealant, wherein a space is enclosed by the sealant and the two glass substrates; thirdly, immersing the filling ports in a liquid crystal material in a vacuum chamber; and finally, introducing gas into the vacuum chamber to make the liquid crystal molecules fill up the space.
The second method is the so-called one-drop-fill (ODF) method. This method comprises the following steps: firstly, printing a sealant on a first glass substrate, wherein the sealant is rectangular and continuous, and a space is enclosed by the sealant and the first glass substrate; secondly, putting liquid crystal molecules into the space drop by drop using a dispenser; and finally, combining a second glass substrate with the first glass substrate and curing the sealant. The ODF process also needs to be performed in a vacuum, at least at the time when the substrates are combined. Thus, in both the first and second methods, a substrate attaching device that can provide a vacuum is necessary.
A typical substrate attaching device utilizes an electrostatic chuck (ESC) and a working table. A first substrate is attached to the ESC by way of electrostatic attraction produced by a voltage applied on the ESC. A second substrate is placed on the working table. The ESC and the working table are moved toward each other until the first substrate is attached to the second substrate. Then the applied voltage is stopped, so that the ESC no longer attracts the first substrate. However, electrostatic charges tend to accumulate on the ESC, and these charges do not immediately dissipate when the applied voltage is stopped. That is, the electrostatic attraction between the first substrate and the ESC does not immediately cease, and it may be difficult to disengage the first substrate from the ESC.
Referring to FIG. 4, this shows another typical substrate attaching device 1. The substrate attaching device 1 comprises a vacuum chamber 11, a first ESC 12, a working table 13, a gas supply 14, a vacuum pump 15, and a controller 16. The first ESC 12 is set substantially in the vacuum chamber 11, and the working table 13 can be set in the vacuum chamber 11. The first ESC 12 comprises a chuck body 121 with a plurality of gas releasing holes 122. The gas releasing holes 122 communicate with a gas pipe 141, so that nitrogen gas can be transferred from the gas supply 14 to a bottom surface of the chuck body 121. A valve 142 is set along the gas pipe 141, to open and close and thereby regulate the flow of nitrogen gas. The vacuum chamber 11 is connected with the vacuum pump 15 through a gas pipe 151. A valve 152 is set along the gas pipe 151, to open and close and thereby regulate the flow of gases. The controller 16 controls the gas supply 14 and the vacuum pump 15 to operate at different times, to feed nitrogen gas and evacuate the inside of the vacuum chamber 11 alternately.
In operation, a substrate attaching method using the substrate attaching device 1 comprises the following steps. Firstly, a second substrate 23 is placed on a working table 13. A sealant 231 is printed on the second substrate 23, and liquid crystal material 232 material is dropped onto the second substrate 23 in a space defined by the sealant 231. Secondly, a first substrate 22 is attached to the chuck body 121 by electrostatic attraction. Thirdly, the first substrate 22 and the second substrate 23 are aligned with each other, and the vacuum pump 15 is operated by the controller 16 to evacuate the vacuum chamber 11. The first ESC 12 is lowered toward the working table 13, so that the first substrate 22 is attached onto the second substrate 23. Finally, referring to FIG. 5, the vacuum pump 15 is stopped by the controller 16. Nitrogen gas is and supplied to the bottom surface of the chuck body 121 by the gas supply 14 under the control of the controller 16. The flowing nitrogen gas can help separate the first substrate 22 from the chuck body 121, so that the combined first and second substrates 22, 23 can be moved out from the vacuum chamber 11.
However, when the gas supply 14 supplies nitrogen gas to the chuck body 121, the vacuum pump of the vacuum chamber 11 stops operating. Thus, a pressure in an inside space of the combined substrates 22, 23 is lower than a pressure in the vacuum chamber 11. Under such conditions, the nitrogen gas may penetrate into the inside space of the combined substrates 22, 23. Bubbles of nitrogen gas may form in the liquid crystal material 232. These can result in impaired performance or even failure of the LCD cell.
What is needed, therefore, is a substrate attaching device that can improve a manufacturing yield of LCD cells. What is also needed is a substrate attaching method that can improve a manufacturing yield of LCD cells.